I put my head all the way back on the nap of my neck, and look absolutely forward, eyes straight darted. It's the hardest part, the eyes like to follow things. In this position, I have my 45°. I take some steps back until the tree's top comes just in the middle of my field of vision. Blinking the eyes help to overcome the following reflex. Then I know the top will (or should at least) fall just 6 ' behind me.
No lean, level ground of course, and no sore neck by sleeping in front of the computer !
If I want to be more accurate, I use the sticks trick, or stick trick.
I guess stacking 10's by eye. Pretty accurate against direct measure after the fell.
Stacking many reference heights is difficult because the eyes (and brain) do that with an angular motion, but the actual height is vertical and in a tangent mode compared to your eyes. The first reference height is (almost) good, but the second one drives your eyes a little too height, and the next adds an even more big error and so on. You have to reduce the reference height each time to follow the tangent progression.
I suppose you are so well used to do that, that it's automatic in your mind.
Mine doesn't succeed in doing this correction and I was wrong each time I tried this method.